“Gasification gas” is a product of gasification, for example coal gasification and pyrolysis of carbonaceous feeds. In particular, the gas produced by pyrolysing and methanating a biomass feed, i.e. a methane based fuel gas, is considered to be a source of renewable energy and thus it offers a promising solution in achieving carbon neutrality for the processing industries. Furthermore, its high methane content makes it ideal for fueling generators and gas engines where its lean-burn characteristic is favorable for meeting emission requirements.
As gasification process takes place, in some cases, in reaction chambers at an operating temperature of above 750° C., the gas extracted from the chambers often carries vaporised long chain hydrocarbons such as tars, as well as other impurities that need to be stripped from the fuel gas before it can be used. For example, the presence of tars in fuel gas reduces its purity, and in some cases, i.e. gas engine applications, leads to inefficiencies, uncontrolled emissions and internal fouling.
Removal of tars from gasification gas/fuel gas is well known in the art. For example, wet scrubbers are commonly used for removing tars by contacting the gas with a scrubber solution, i.e. water, so that on contact any vaporised tars are condensed and retained in the scrubber solution (see examples given in US20140131622-Winter et al.), resulting in a high purity fuel gas. U.S. Pat. No. 4,324,643 (Durai-Swamy) and U.S. Pat. No. 4,101,412 (Choi) provide some further examples on extracting the tars from the tar-rich scrubber solution, including liquid separation techniques and fractionation columns. The use of wet scrubbers offer a simple solution for gas cleaning, but downstream tar extraction shown in these examples may prove to be expensive and can cause potential harm if the scrubber solutions are not properly treated. Alternatively, the tars may be directly condensed out of the fuel gas without the use of scrubber solution. KR20110137977 (Korean Institute of Energy Research) teaches a centrifugal condenser where fuel gas flowing through a revolving annulus are continuously cooled by the surrounding cooling fluids. On cooling the vaporised tars are condensed out of the fuel gas and flow along an angled wall before leaving the annulus at a liquid exit with the aid of centrifugal force, leaving a stream of conditioned and purified gas to be purged at a gas exit. However processing temperature greatly affects tar viscosity and so inadequate control of the cooling temperature, i.e. overcooling, may lead to thickened tars and thus blockage at the liquid exit, i.e. the centrifugal force imparted by the revolving annulus may not be sufficient to expel the thickened tars out of the revolving annulus.
Tars are long chain hydrocarbons that are the condensable fractions in process gases created by industrial processes. Typically tars with larger molecular weights have higher dew points. These tars typically consist of many aromatic rings for example heavy polyaromatic hydrocarbons. Tars with lower molecular weights, often with fewer aromatic rings, usually have lower dew points. In general tars with higher dew points will have higher viscosities at a specific temperature than those with lower dew points. When in mixture these tars can form a viscous paste which is undesirable and problematic in any gas treatment process. Using the system taught in KR20110137977 for tar removal would inevitably result in great inconsistency in viscosity, which affects heat transfer and drainage performance. For example, in KR20110137977 where the coolant is supplied at a single cooling temperature, the tar mixture containing tars with high dew points significantly thicken the overall mixture, preventing effective draining.
As a result, a gas conditioning unit that is able to efficiently and reliably remove tars from a gasification gas/fuel gas is highly desirable.